1. Field of the Invention
The present invention relates to a reflection-type or transflective-type liquid crystal display device which can perform display by utilizing reflected light.
2. Description of the Related Art
Liquid crystal display devices (LCDs) include the transmission-type LCD which utilizes backlight from behind the display panel as a light source for displaying, the reflection-type LCD which utilizes reflected light of external light, and the transflective-type LCD (reflection/transmission-type LCD) which utilizes both reflected light of external light and backlight. The reflection-type LCD and the transflective-type LCD are characterized in that they have smaller power consumptions than that of the transmission-type LCD, and their displayed images are easy to see in a bright place. The transflective-type LCD is characterized in that their displayed images are easier to see than that of the reflection-type LCD, even in a dark place.
FIG. 17 is a cross-sectional view showing an active matrix substrate 100 in a conventional reflection-type LCD (e.g., Japanese Laid-Open Patent Publication No. 9-54318).
As show in this figure, the active matrix substrate 100 includes an insulative substrate 101, as well as a gate layer 102, a gate insulating layer 104, a semiconductor layer 106, a metal layer 108, and a reflective layer 110, which are stacked on the insulative substrate 101. After being stacked on the insulative substrate 101, the gate layer 102, the gate insulating layer 104, the semiconductor layer 106, and the metal layer 108 are subjected to etching by using one mask, thus being formed so as to have an island-like multilayer structure. Thereafter, the reflective layer 110 is formed on this multilayer structure, whereby a reflection surface 112 having roughened portions is formed. Although not shown, transparent electrodes, a liquid crystal panel, a color filter substrate (CF substrate), and the like are formed above the active matrix substrate 100.
FIG. 18 is a cross-sectional view of a conventional transflective-type liquid crystal display device (e.g., Japanese Laid-Open Patent Publication No. 2005-277402).
As shown in this figure, in the conventional transflective-type liquid crystal display device, an interlayer insulating film 204 is formed above a drain electrode 222 of a switching element (TFT) 203, and a galvanic corrosion preventing film 205, a reflection electrode film 206, and an amorphous transparent electrode film 218 are stacked on the interlayer insulating film 204. The region where the reflection electrode film 206 is formed is a reflection region of the transflective-type liquid crystal display device. Roughened portions are formed in an upper portion of the interlayer insulating film 204 in the reflection region. Corresponding to these roughened portions, roughened portions are also formed on the galvanic corrosion preventing film 205, the reflection electrode film 206, and the amorphous transparent electrode film 218.
In the aforementioned active matrix substrate 100, portions of the reflective layer 110 are formed so as to reach the insulative substrate 101 in portions where the gate layer 102 and the like are not formed (i.e., portions between the islands, hereinafter referred to as “gap portions”). Therefore, in the gap portions, the surface of the reflection surface 112 is recessed in the direction of the insulative substrate 101, thus forming a surface having deep dents (or recesses).
In the reflection-type liquid crystal display device or the transflective-type liquid crystal display device, in order to perform bright display by utilizing reflected light, it is necessary to allow incident light entering from various directions to be reflected by the reflection surface 112 more uniformly and efficiently over the entire display surface. For this purpose, it is better if the reflection surface 112 is not completely planar but has moderately roughened portions.
However, the reflection surface 112 of the aforementioned active matrix substrate 100 has deep dents. Therefore, light is unlikely to reach the reflection surface located on the bottoms of the dents, and even if at all light reaches there, the reflected light thereof is unlikely to be reflected toward the liquid crystal panel, thus resulting in a problem in that the reflected light is not effectively used for displaying. Furthermore, there is a problem in that, since many portions of the reflection surface 110 have a large angle relative to the display surface of the liquid crystal display device, the reflected light from those portions is not effectively utilized for displaying.
FIGS. 19A and 19B are diagrams showing a relationship between the tilt of the reflection surface 112 and the outgoing angle of reflected light. FIG. 19A shows a relationship between an incident angle a and an outgoing angle S when light enters a medium b having a refractive index Nb from a medium a having a refractive index Na. In this case, according to Snell's Law, the following relationship holds true.Na* sin α=Nb* sin β
FIG. 19B is a diagram showing a relationship between incident light and reflected light when incident light perpendicularly entering the display surface of an LCD is reflected from a reflection surface which is tilted by θ with respect to the display surface (or the substrate). As shown in the figure, the incident light perpendicularly entering the display surface is reflected from the reflection surface which is tilted by angle θ with respect to the display surface, and goes out in a direction of an outgoing angle φ.
According to Snell's Law, results of calculating the outgoing angle φ according to Snell's Law with respect to each angle θ of the reflection surface are shown in Table 1.
TABLE 1Θφ90 - φ009026.00612183.99388412.0496777.95033618.1718171.82819824.4221265.577881030.8658859.134121237.5970952.402911444.7655445.234461652.6438237.356181861.8454328.154572074.6185715.3814320.579.7654210.2345820.681.127578.87243220.782.733157.26684820.884.803115.1988820.988.850361.14963720.90589.799140.200856
The values in this Table are calculated by assuming that air has a refractive index of 1.0 and the glass substrate and the liquid crystal layer have a refractive index of 1.5. As shown in Table 1, when the angle θ of the reflection surface exceeds 20 degrees, the outgoing angle φ becomes very large (i.e., 90-φ becomes very small), so that most of the outgoing light does not reach the user. Therefore, even if roughened portions are provided on the reflection surface of the reflective layer, it is necessary to ensure that the angle θ is 20 degrees or less in greater portions of the reflection surface in order to effectively use the reflected light.
Since the reflection surface 112 of the aforementioned active matrix substrate 100 has many portions which are greater than 20 degrees, reflected light is not very effectively used for displaying. In order to solve this problem, it might be possible to form an insulating layer under the reflective layer 110, and form the reflective layer 110 over the insulating layer. However, in this case, a step of forming an insulating layer, and a step of forming contact holes for connecting the reflective layer 110 to the drains of TFTs in the insulating layer are needed, thus resulting in a problem of an increase in the material and the number of manufacturing steps.
Moreover, the aforementioned conventional transflective-type liquid crystal display device requires a step of stacking the interlayer insulating film 204 on the drain electrode 222 and then forming roughened portions in an upper portion thereof, and further a step of stacking thereupon the galvanic corrosion preventing film 205, the reflection electrode film 206, and the amorphous transparent electrode film 218. Thus, the conventional transflective-type liquid crystal display device also has a problem in that the material and number of steps are increased for forming the reflection region.
Furthermore, in the conventional transflective-type liquid crystal display device, roughened portions are formed on the surface of the amorphous transparent electrode film 218, which is in contact with the liquid crystal layer 211. Therefore, an electric field which is applied across the liquid crystal layer 211 does not become uniform, and it is difficult to control the orientation of the liquid crystal in the reflection region uniformly in a desired direction. Moreover, a slope conforming to the end shape of the interlayer insulating film 204 is formed at an end of the amorphous transparent electrode film 218. There is also a problem in that this slope disturbs the orientation of the liquid crystal near the end of the reflection region.